This invention is in the field of data communications, and is more specifically directed to optimization of discrete multitone modulation communications by selection of an optimum communications mode.
Digital Subscriber Line (DSL) technology has become one of the primary technologies in the deployment of high-speed Internet access in the United States and around the world. As is well known in the art, DSL communications are carried out between a central office (CO) location, operated by a telephone company or an Internet service provider, and individual subscribers, using existing telephone “wire” facilities. Typically, some if not all of the length of the loop between the CO and the customer premises equipment (CPE) is implemented by conventional twisted-pair copper telephone wire. Remarkably, modem DSL technology is able to carry out extremely high data rate communications, even over reasonably long lengths (e.g., on the order of 15,000 feet) of twisted-pair wire, and without interfering with conventional voiceband telephone communications.
Modem DSL communications achieve these high data rates through the use of multicarrier modulation (MCM) techniques, also referred to as discrete multitone modulation (DMT), by way of which the data signals are modulated onto frequencies in a relatively wide frequency band that resides well above the telephone voice band, and that is subdivided into many subchannels. The data symbols modulated onto each subchannel are encoded as points in a complex plane, according to a quadrature amplitude modulation (QAM) constellation. The number of bits per symbol for each subchannel (i.e., the “bit loading”), and thus the number of points in its QAM constellation, is determined according to the signal-to-noise ratio (SNR) at the subchannel frequency, which depends on the transmission channel noise and the signal attenuation at that frequency. For example, relatively noise-free and low attenuation subchannels may communicate data in ten-bit to fifteen-bit symbols, represented by a relatively dense QAM constellation with short distances between points in the constellation. On the other hand, noisy channels may be limited to only two or three bits per symbol, allowing a greater distance between adjacent points in the QAM constellation. High data rates are attained by assigning more bits (i.e., a more dense QAM constellation) to subchannels that have low noise levels and low signal attenuation, while subchannels with poorer SNRs can be loaded with a fewer number of bits, or none at all.
The most popular class of DSL communications protocols are referred to generically as asymmetric DSL (“ADSL”). Under ADSL, in this generic sense, frequency-division duplexing (FDD) carries out “downstream” communications from the telephone company central office (“CO”) to customer premises equipment (“CPE”) in one frequency band of the spectrum, and carries out “upstream” communications from the CPE to the CO in another, non-overlapping, frequency band. The asymmetry of asymmetric DSL refers to the assignment of a wider and higher-frequency band to downstream communications, and a narrower, lower-frequency, band to upstream communications. As a result, according to these technologies, the downstream data rate is typically much greater than the upstream data rate, except in those cases in which the loop length is so long that the downstream frequency band is mostly unusable.
Various DSL standards have been adopted in recent years. For example, ADSL under the so-called G.lite standard described in Splitterless Asymmetric Digital Subscriber Line (ADSL) Transceivers, ITU-T Recommendation G.992.2 (International Telecommunications Union, 1999) utilizes thirty-two upstream subchannels and 128 downstream subchannels, each subchannel having a bandwidth of 4.3125 kHz. As such, the bandwidth utilized under G.lite ADSL extends up to about 552 kHz. Newer DSL technologies provide higher data rates by variations of the DMT scheme of ADSL. DSL service according to the well-known “ADSL” standard (used in a specific sense relative to the standard Asymmetric digital subscriber line transceivers (ADSL), ITU-T Recommendation G.992.1 (International Telecommunications Union, June 1999)), presently dominates much of the commercial DSL service in the United States, and utilizes thirty-two upstream subchannels and 256 downstream subchannels, extending the bandwidth to 1.104 MHz. The “ADSL2” standard increases the available data rate relative to the ADSL standard, but without increasing the number of subchannels and bandwidth; the performance improvements are attained under ADSL2 by way of improved modulation efficiency, reduced framing overhead, higher coding gain, an improved initialization procedure, and enhanced signal processing algorithms. The ADSL2 standard is described in Asymmetric digital subscriber line transceivers 2 (ADSL2), ITU-T Recommendation G.992.3 (International Telecommunications Union, July 2002). Under the relatively new “ADSL2+” standard, the downstream data bandwidth is extended to 2.2 MHz using 512 subchannels of 4.3125 kHz., as described in Asymmetric Digital Subscriber Line (ADSL) transceivers—Extended bandwidth ADSL2 (ADSL2+), Recommendation G.992.5 (International Telecommunications Union, May 2003). And additional DSL standards are also known in the art, including such protocols as very high bit-rate DSL (“VDSL”), which provides extremely high data rates via up to 4096 subchannels, at frequencies extending up to 30 MHz.
These multiple standards have each encountered substantial deployment in the field, with service providers and clients each tending toward higher data capacity where economically feasible. However, it is economically efficient for equipment manufacturers to manufacture and market equipment that can operate according to multiple standards, to provide customers with the flexibility of deployment and to reduce inventory pressures. As such, so-called multi-mode DSL transceiver equipment, capable of carrying out DSL communications according to any one of a number of standards, are known in the art.
By way of further background, so-called “automode” DSL transceiver equipment for deployment at central office (CO) or service area interface (SAI) locations in DSL communications networks are known. According to this automode approach, the CO transceiver effects initialization sequences with a client premises equipment (CPE) transceiver according to each of multiple DSL standards, measuring the actual data rate under each standard, and then selects the communications standard, or mode, that provided the highest data rate for that subscriber and loop. This approach obviously requires long training times in establishing a communications session (which, when multiplied by the number of sessions to be supported, results in substantial overhead), and also requires investment at the CO or SAI in order to support such automode.